The invention relates to a two-stroke engine and especially a drive engine in a portable handheld work apparatus including a motor chain saw, a brushcutter, cutoff machine or the like.
In a known two-stroke engine of this kind, air is supplied via the air channel and the check valve to each transfer channel or the transfer channel close to the outlet during the induction phase. During a gas exchange, the air, which is stored in the transfer channel, is pushed into the combustion chamber by the air/fuel mixture following on from the crankcase. In this way, scavenging losses are reduced and therefore the quality of the exhaust gas is improved.
For metering the fuel-free gas, which is to be supplied to the transfer channel, a throttle is mounted in the air channel and this throttle is configured as a throttle flap. The throttle flap is purposefully coupled to the carburetor throttle flap in order to meter the air, which is to be supplied to the transfer channel, in correspondence to the operating state of the two-stroke engine. For this purpose, the throttle flap has to be adapted to the throttle channel; especially, it should be ensured that, when starting the engine, virtually no fuel-free gas can enter into the transfer channel via the air channel. This is so because this can lead to a leaning of the mixture and therefore to starting difficulties associated therewith.
It is an object of the invention to improve a two-stroke engine so that the precise metering of the fuel-free gas into the transfer channels is ensured with a minimum of constructive complexity.
The two-stroke engine of the invention includes an engine in a portable handheld work apparatus. The two-stroke engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; at least one transfer channel connecting the crankcase to the combustion chamber; the transfer channel having a first end defining an entry window opening into the combustion chamber and a second end opening into the crankcase; an air filter having an air filter housing defining a clean space; a choke throttle unit for conducting essentially fuel-free air from the air filter; an air channel connected to the transfer channel for supplying the essentially fuel-free gas flow thereto from the choke throttle unit; a mixture-preparation unit for supplying an air/fuel mixture; the mixture-preparation unit being mounted downstream of the air filter; an inlet channel downstream of the mixture-preparation unit for conducting the air/fuel mixture into the crankcase; the choke throttle unit including: an independent choke housing defining choke channel communicating with the air channel; and, a throttle element adjustably mounted in the choke channel; and, the choke housing being attached to the air filter housing and the choke channel being connected to the clean space of the air filter upstream of the choke element.
The adjustable throttle element is arranged in a throttle channel of an independent throttle housing which can be configured to the requirements of the adjustment accuracy. The throttle channel is connected to the clean air side of the air filter upstream of the throttle flap so that a separate air filter is unnecessary for the air which is advance-stored in the transfer channel. The air filter is connected forward of the mixture-preparation device. The needed air volume of the engine is made available for each operating point thereof via the air filter.
Preferably, the throttle housing is made of plastic and is configured especially as a plastic injection molded part. The throttle element is mounted in a pipe or tube section made of metal which defines a channel in order the ensure manufacture as a mass-produced item while guaranteeing high accuracy of the throttle flap fit. The pipe section can be injection molded in the throttle housing. In this way, the throttle flap can be fitted in the pipe section so as to be approximately seal tight in the closed position and can effectively block the fuel-free supply of gas into the transfer channel for the starting case.
The throttle flap housing is purposefully attached to the housing base of the air filter and a corresponding connection support can be provided for this purpose. The throttle housing is advantageously configured as one piece with the air filter and especially with the housing base of the air filter housing.